Hybrid heterostructure of transition metal dichalcogenides as potential photocatalyst for hydrogen evolution

Highlights

• There is a synergy effect between interlayer and intralayer in (WS₂-MoS₂)/WS₂ heterostructure.

• Uneven electrostatic potential distribution leads to a build-in electric field at interfaces.

• Type-II band alignment is observed for (WS₂-MoS₂)/WS₂ hybrid heterostructure.

• (WS₂-MoS₂)/WS₂ hybrid heterostructure is a potential photocatalyst for hydrogen evolution.

• Band gap of (WS₂-MoS₂)/WS₂ hybrid heterostructures is reduced under tensile strain.

Abstract

Transition metal dichalcogenides (TMDCs) heterostructure with property diversity is of scientific and technological interest for nanoelectronics and photoelectronic devices. Herein, we investigate electronic properties of TMDCs hybrid heterostructures, i.e., (WS₂-MoS₂)/WS₂, (WS₂-MoS₂)/MoS₂ and WS₂/(WS₂-MoS₂)/WS₂ heterostructures. From analyses of energetics, charge transfer and interaction strength, we find there is a synergistic effect, i.e., strong intralayer and moderate interlayer interaction, in these hybrid heterostructures. Electrostatic potential analysis implies there is a build-in electric field at interfaces of these hybrid heterostructures, which would influence the migration of photoinduced carriers. Combination of build-in electric field with band offset calculations, separation mechanism of photoinduced carriers is discussed. Our calculated results show that (WS₂-MoS₂)/WS₂ and WS₂/(WS₂-MoS₂)/WS₂ hybrid heterostructures are potential type-II photocatalyst for hydrogen evolution. (WS₂-MoS₂)/MoS₂ heterostructure is a candidate material for infrared laser light emitting devices due to its type-I band alignment. Finally, the influence of strain on band gap of (WS₂-MoS₂)/WS₂ hybrid heterostructure is also investigated. We believe our study suggests potential direction for fabrication of novel electronic and photoelectric devices based on TMDCs heterostructures.

Introduction

Inspired by the research of graphene, more and more two-dimensional (2D) materials with different physical and chemical properties have been successfully prepared. On basis of 2D materials, people integrated them with different dimensional materials into a mixed dimensional heterostructures, namely 2D+nD (n = 0,1,3) heterojunctions [1], [2], [3], to meet various device requirements. Such hybrid heterojunctions possess the diversity of physical and chemical property as well as structure, not only providing theoretical models for basic scientific research, but also opening up new directions for device design [1], [2], [3]. Among these hybrid heterojunctions, semiconductor heterostructures, such as transition metal dichalcogenides (TMDCs) heterostructures, are important building blocks for important devices such as solar cells [3], light-emitting diodes [4], p−n rectifying diodes [5], photovoltaic devices [6], photoelectronic devices [7] and high-electron-mobility transistors [8]. Analogous to the traditional heterojunctions, layered TMDCs heterostructures can be prepared by assembling individual single layer into functional heterostructures [9], [10] with atomically sharp interfaces, digitally controlled components, and free lattice constraint. Moreover, atomically thin TMDCs heterojunctions with various geometries and band alignments are of scientific and technological interest for exploration of the next generation flexible nanoelectronics [2].

In past few years, studies of TMDCs have attracted considerable attentions. In addition to their intrinsic properties [11], [12], research interest in TMDCs heterostructures [13], [14], [15], [16], [17], [18], [19], [20], [21], [22] continues to grow rapidly and broadens the range of applications. Substantial theoretical [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29] and experimental studies [4], [22], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39] have been performed to investigate the nature of interaction between different layers of vertical TMDCs heterostructures or different components of lateral TMDCs heterostructures. Many fundamental issues related to fabricated techniques and properties of TMDCs heterostructures, such as synthesis and preparation method [30], interlayer coupling [31], band-to-band tunneling [32], valleytronics [33], band alignment [34] and ultra-fast charge transfer [35], have been the focus of the studies.

Nevertheless, TMDCs heterostructures based on molybdenum sulfide (MoS₂) and tungsten sulfide (WS₂) have potential applications in electronic devices. For example, ultra-fast charge transfer has been observed [36] in vertical MoS₂/WS₂ heterojunction, which is important for photoelectric conversion efficiency and response speed of devices [37]. Experimental studies show that ultra-fast charge transfer from MoS₂ to WS₂ layer can enable their novel van der Waals (vdW) heterostructures to use for 2D devices of optoelectronics and light harvesting [6]. It has been found that optical excited plasma oscillation of MoS₂/WS₂ heterogeneous interface can be promoted by interfacial charge transfer [13]. Besides, separation process of electron-hole pairs in vertical MoS₂/WS₂ heterojunction has been found to depend on the values of coupling matrix element of the hole states between layers in vertical direction [13]. On the other hand, by using GW method combined with Bethe-Salpeter equation (BSE), band alignment of MoS₂/WS₂ heterojunction has been studied, and it is found that MoS₂/WS₂ heterojunction belongs to type-II band alignment [14]. This finding is conducive to the development and application of ultra-thin photovoltaic devices. An anomalous photoluminescence quenching has been observed in WS₂/MoS₂ heterostructure from recently experiments [40], demonstrating its great potential application in future optoelectronic devices. Another experiment has clarified the origin of transfer rate robustness against interlayer stacking configurations in MoS₂/WS₂ heterostructures, facilitating its application in high-efficient optoelectronic and photovoltaic devices [36].

In addition, in-plane and out-of-plane heterostructures based on MoS₂ and WS₂ also show excellent performance in catalytic hydrogen evolution. Shi et al. [41] successfully prepared MoS₂/WS₂ heterostructures with different stacking methods, and found that different stacking sequences showed different effects on photocatalytic hydrogen evolution. They also found that MoS₂/WS₂ heterojunction is a type-II photocatalyst and electrons and holes are enriched in MoS₂ and WS₂ respectively, to enhance the hydrogen evolution performance. It has been reported that MoS₂/WS₂ vdw heterostructure exhibits high hydrogen evolution reaction (HER) performance due to active sulfur edge sites or defect sites [42]. Few-layer in-plane WS₂–MoS₂ heterostructures can also be used as high-efficiency photocatalyst [43] due to rapid electron-hole separation upon irradiation. Such an improved photocatalytic H₂ evolution from pure two-dimensional in-plane heterostructures has the potential to deliver outstanding photocatalysts for solar energy conversion as well as degradation of chemical fuels. Furthermore, hybrid heterostructures based on TMDCs have been observed in experiments and have been proved to be a potential photocatalyst for hydrogen evolution. For instance, a noble-metal-free nanohybrid CdS/WS₂–MoS₂ has been fabricated and has been observed to exhibit high hydrogen evolution efficiency and remarkable stability, which is due to ultrafast separation of photogenerated charge carriers and transport between CdS nanorods and WS₂–MoS₂ nanosheets [44]. Furthermore, nanohybrid electrocatalyst WS₂–MoS₂/graphene exhibit outstanding HER performance mainly owe to the plethora of widely exposed electrochemically active sites [45]. Therefore, heterostructures based on WS₂ and MoS₂ have important applications in the field of hydrogen evolution and their construction is feasible and successful in experiments.

Although lots of investigations have been done on TMDCs heterojunctions, the structures and properties of pure hybrid TMDCs heterojunctions are poorly understood. More importantly, heterostructures based on TMDCs have been studied for years and type-II band alignment has been known for long time [46], the migration mechanism of photoinduced carriers and regional selectivity of catalytic reaction in pure hybrid TMDCs heterostructure have never been mentioned either experimentally or theoretically. We also would like to point out that migration mechanism of photoinduced carriers is important for photocatalytic process, especially for photocatalytic water splitting based on TMDCs heterostructures. In fact, such pure hybrid TMDCs heterostructure have been observed in several experimental studies [1], [2], [3]. For example, interface in 2D bi-layer-monolayer TMDCs heterostructures can be formed by various fabricated methods [9], [10], [30], [34], [39], leading to an abrupt change of electronic properties at bilayer-monolayer interface [2]. It has been shown that such partial decoration geometry can induce an oscillatory electrostatic potential distribution [47], [48], [49] and further influence adsorption and diffusion of foreign elements on its surface. In addition, pure hybrid heterostructures formed by organic molecules on 2D lateral heterostructures have been shown to hold potential in achieving solar energy conversion [3]. These studies indicate that 2D TMDCs materials can be composed with other different dimensional materials or motifs to form hybrid heterojunction. Hybrid heterojunction not merely enriches diversity of materials, but also provides special properties, such as undergoing sharp changes in potential energy barrier or density of states, thus providing abundant physical properties. However, detailed theoretical research on properties and regulation of 2D hybrid heterojunction materials is still lacking. The effects of interfacial interaction on properties of 2D hybrid heterojunction are still unclear. Especially, the microscopic photocatalytic mechanism in hybrid TMDCs heterostructure has never been even noticed at all. Moreover, all previous theoretical interpretations of various interesting phenomena on TMDCs heterostructures also overlooked the effect of electric field on the migration of photoinduced carriers.

In this study, we systematically study interaction between separated components, electrostatic potential distribution and band alignment of WS₂-MoS₂, MoS₂/WS₂, (WS₂-MoS₂)/WS₂, (WS₂-MoS₂)/MoS₂ and WS₂/(WS₂-MoS₂)/WS₂ heterostructures from first-principles calculations. Our calculated results indicate that there are strong chemical bonds between lateral interface and weak vdW interaction between vertical interface in (WS₂-MoS₂)/WS₂, (WS₂-MoS₂)/MoS₂ and WS₂/(WS₂-MoS₂)/WS₂ hybrid heterostructures. We find uneven electrostatic potential distribution leads to a build-in electric field at interfaces in these hybrid heterostructures. Besides, band offsets combined with build-in electric field, are calculated to analyze microscopic separation mechanism of photoinduced electrons and holes. Moreover, we find band gap can be tuned by applying strains. Our results reveal that (WS₂-MoS₂)/WS₂ and WS₂/(WS₂-MoS₂)/WS₂ hybrid heterostructures exhibit decent band edges, which is a potential photocatalyst for hydrogen evolution in future. While (WS₂-MoS₂)/MoS₂ hybrid heterostructure is a type-I heterostructure, which is potential candidate material for infrared laser emitting devices.